It is known that the concentration of certain components in gas mixtures can be determined by means of polarographic sensors. This is done, for example, in the case of exhaust gases of internal-combustion engines which are run using gaseous or liquid fuels, such as otto engines or diesel engines. It is desirable to know the content, in the exhaust gas, of oxygen and/or combustible components such as hydrogen, hydrocarbons and carbon monoxide, because these values permit inferences concerning the operational state of the engine, i.e. make it possible to distinguish between an operating mode with "lean" or "rich" air-fuel mixes.
The knowledge of the operational state forms the basis for controlling interventions for the purpose of optimizing, in a particular case, the composition of the air-fuel mix. In lean air-fuel mixes, oxygen is present in a stoichiometric excess, and accordingly, significant concentrations of oxygen are measured in the exhaust gas, while combustible components occur, if at all, in minor amounts. The opposite is the case if the engine is run on rich air-fuel mixes. In that case, considerable amounts of combustible components are still present in the exhaust gas, while oxygen occurs, if at all, at a minor concentration. A numerical measure for distinguishing between lean and rich mixes is the lambda (.lambda.) number which represents equivalent ratio of oxygen to combustible fractions. It is &gt;1 in lean mixes, &lt;1 in rich mixes and =1 if oxygen and combustible components are present in a stoichiometric ratio, which is what is generally aimed for.
Polarographic probes are based on measuring the limiting current of a pump cell. In order to measure oxygen in exhaust gases from lean mixes ("lean exhaust gases") there is arranged, upstream of the cathode, a diffusion barrier which makes it so difficult for the oxygen to reach the cathode that even at an only moderate pump voltage all of the molecular oxygen is reduced virtually immediately to 0.sup.2- ions which migrate through the electrolyte of the pump cell and are again discharged, at the anode, to give molecular oxygen. The current cannot be increased further by a higher pump voltage; a limiting current flows whose intensity virtually depends only on the oxygen concentration in the exhaust gas and on the characteristics of the diffusion barrier, especially on its layer thickness and porosity. If the probe is calibrated with reference gases, it is possible to establish an unambiguous relationship between the intensity of the limiting current and the oxygen concentration.
In the case of exhaust gases of rich air-fuel mixes ("rich exhaust gases") the combustible fractions, contained therein in a significant amount, are oxidized anodically. Here, too, it is possible to measure a concentration-dependent limiting current if the diffusion of the combustible fractions to the anode is impeded. This can be achieved, in conventional measuring probes, by reversing the polarity of the pump cell, i.e., interchanging anode and cathode. Therefore, only one diffusion barrier is present in front of one of the electrodes which, for measurements in lean gases, is connected as the cathode and for measurements in rich gases is connected as the anode. A drawback of these probes is that their polarity has to be reversed for the purpose of the rich-lean distinction, as this requires additional measuring and control effort. Moreover, the probes do not operate reliably immediately after a switch-over operation, because a steady state at the electrodes whose polarity has been reversed is established only after a certain time.
EP-B1-0 194 082 disclosed a sensor with two cells, which has the following elements:
(a) a pump cell with a first solid electrolyte and, disposed thereon, a first and a second porous electrode, PA1 (b) an "electrochemical sensor cell" with a second solid electrolyte and a third and fourth electrode, the third electrode being disposed close to the first electrode of the pump cell, PA1 (c) a diffusion resistance which impedes access of the gas to be measured to the first and third electrode, PA1 (d) a device for applying a pump current between the first and the second electrode of the pump cell, PA1 (e) a device for measuring the potential difference (or electromotive force) between the third and the fourth electrode of the "electrochemical sensor cell", and finally PA1 (f) a device for applying an auxiliary pump current between the third electrode of the "electrochemical sensor cell" and another electrode.
This sensor, of complicated design, thus emits (see feature (e)) a control signal determined potentiometrically. Moreover, the transition between rich and lean requires an electrode polarity reversal, with the same drawbacks as described previously.